and tumor associated blood vessels originate as precursors in the bone marrow. Recent studies, using flow cytometry have demonstrated that these precursors, as well as mature endothelial cells, can be detected in the circulation of patients with cancer and that the levels of these cells are significantly higher in patients with malignancy than normal controls. Furthermore, successful therapy for the malignant process may affect the number of activated circulating endothelial cells (CECs) in the peripheral blood. These data suggest that measurements of CECs and circulating endothelial cell precursors (CEPs) in the peripheral blood of cancer patients may be a used as a surrogate marker for the efficacy of anti-cancer strategies, particularly those that target tumor vasculature. The Flow Cytometry Unit provides collaborative support for an NCI protocol designed to determine if flow cytometric enumeration of CECs can be used to measure levels of CECs and CEPs in the peripheral blood of patients with cancer or of patients undergoing surgical interventions, and to determine if they can be used to follow response to standard therapy or anti-angiogenic agents or disease progression across a variety of histologies and treatments. This type of analysis is not standard in the community and is only available in a few academic centers worldwide. Investigator from other countries have visited in the laboratory to learn this specialized technology so that their medical institutes can participate in NCI sponsored trials involving measurement of CECs and CEPs.